Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
In ink jet recording processes, it is necessary that the inks being used meet various performance requirements. Such performance requirements are generally more stringent than those for other liquid ink applications, such as for writing instruments (e.g., a fountain pen, felt pen, etc.). In particular, the following conditions are generally required for inks utilized in ink jet printing processes:    (1) The ink should possess physical properties such as viscosity, surface tension, and electric conductivity matching the discharging conditions of the printing apparatus, such as the driving voltage and driving frequency of a piezoelectric electric oscillator, the form and material of printhead orifices, the diameter of orifices, etc;    (2) The ink should be capable of being stored for a long period of time without causing clogging of printhead orifices during use;    (3) The ink should be quickly fixable onto recording media, such as paper, film, etc., such that the outlines of the resulting ink dots are smooth and there is minimal blotting of the dotted ink;    (4) The printed image should be of high quality, such as having a clear color tone and high density, have high gloss and high color gamut;    (5) The printed image should exhibit excellent waterfastness (water resistance) and lightfastness (light resistance);    (6) The printed (ink) images should have good adhesion to the surface of image receiving elements and should be durable and highly resistant to physical and mechanical scratches or damages    (7) The ink should not chemically attack, corrode or erode surrounding materials such as the ink storage container, printhead components, orifices, etc;    (8) The ink should not have an unpleasant odor and should not be toxic or inflammable; and    (9) The ink should exhibit low foaming and high pH stability characteristics.
The inks used in various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant that is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic cosolvents. Each individual dye molecule is surrounded by molecules of the carrier medium. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor lightfastness. When water is used as the carrier medium, such inks also generally suffer from poor waterfastness.
Pigment-based inks have been gaining in popularity as a means of addressing these limitations. In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers that serve to keep the pigment particles from agglomerating and/or settling out.
Pigment-based inks suffer from a different set of deficiencies than dye-based inks. One deficiency is that pigment-based inks interact differently with specially coated papers and films, such as transparent films used for overhead projection and glossy papers and opaque white films used for high quality graphics and pictorial output. In particular, it has been observed that pigment-based inks produce imaged areas that are entirely on the surface of coated papers and films, which result in images that have poor dry and wet adhesion properties and can be easily smudged. In recent years, ink jet receivers have been developed to have both high gloss and high porosity to give fast drying capabilities. However, scratch mark smudges are more visible on high gloss receivers. In order to provide a pigmented ink composition that can be used in printing images on an ink jet porous glossy, receiving element having acceptable durability and smudging resistance, polymers are often used as additive in pigmented inks.
U.S. Pat. No. 6,533,408 discloses classes of water-dispersible polymers used in pigmented ink for both durability and ozone stability improvement, including aqueous polyurethane dispersion. However, not all polymers can be jetted from inkjet printhead easily and reliably.
Addition type polymers have been commonly used in pigment-based inks, as an additive to pigment dispersions to improve print durability. The polymer is usually not ionically or physically associated with pigment dispersion in ink till printed on an ink jet receiver. U.S. Pat. No. 4,597,794 teaches the use of a polymer in pigment based inkjet ink, the polymer has both ethylenically unsaturated carboxylic acid substituent as a hydrophilic portion and an aromatic ring substituent as a hydrophobic portion, and wherein the said hydrophilic portion constitutes from 2 to 40 weight percent of said polymer, and the molecular weight of the polymer prefers to be 1,000 to 100,000. U.S. Pat. No. 5,106,417 claims the use of polyacrylic resins containing carboxylic acid group in pigmented ink to improve image quality and water resistance. Preferred acrylic resins consist of ethyl acrylate, methyl methacrylate and acrylic or methacrylic acid, having acid number of 50-300, and weight average molecular weight (Mw) of 30,000 to 70,000. U.S. Pat. No. 5,172,133 teaches the use acrylic acid-ethyl acrylate-styrene copolymers in pigmented ink jet ink to improve storage stability and clogging resistance. U.S. Pat. No. 5,883,157 teaches the use of styrene-maleic anhydride copolymer in ink jet ink to improve image quality and dry time on plain paper, and resistance to printhead clogging. The mean molecular weight of styrene-maleic anhydride copolymer is from 1000 to 10,000. U.S. Pat. No. 5,913,971 claims the use of a terpolymer obtained from acrylic acid, styrene and α-methyl styrene and has an average molecular weight of 2,000 to 8,000 and an acid number of 90 to 130 in pigment based ink jet ink to improve print durability while maintaining good storage stability. WO 9,516,002 also teaches the use of styrene-acrylic acid copolymer ammonium salt in black ink to improve jettability from an inkjet printhead and high print density.
A general phenomenon has been noticed by the inventors that as acid number of the polymer is increased and molecular weight of the polymer is decreased in order to get satisfactory and uniform jettability from an ink jet printhead, the print durability is compromised, especially if the printed image has been wet with aqueous fluids. The deficiency is due to the fact that polymer has increased water solubility. However, when a polymer having increased molecular weight and decreased acid number is used, image defects due to lack of reliable jettability is noticed, resulting in density decrease in printing direction across the page.
Due to the low cost of materials made from addition polymerization, it is desirable to find an optimum polymer composition to be used in an ink jet ink so that printed images on an ink jet receiving element have improved physical durability such as scratch and smudging resistance while maintaining reliable jettability from ink jet printheads to produce quality images at all time.